Neurons possess prominent intracellular stores of Ca2+ in specialized endoplasmic reticulum. These stores release Ca2+ that acts as a second messenger. Upon depletion, stores must then refill, and recent studies have identified a molecular pathway required for Ca2+ store refilling in the process termed capacitive Ca2+ entry (CCE). The ER resident protein Stimi senses depletion of Ca2+ stores, whereupon it forms clusters within the ER membrane and its C-terminus spans the cytosol to activate membrane ion channels of the Oral family. Orai-Stim1 and CCE play a critical role in the physiology of many cell types. Here, we will examine the role of Stimi and Orai in neural physiology using newly developed conditional genetic deletion models. Aim 1 will examine the role of Stimi and Orai in NMDA-dependent receptor function. Preliminary studies indicate Stimi is required for NMDA receptor dependent long-term depression, and suggest a role for Stimi in activation of calcineurin. Experiments will image Ca2+ dynamics in acute tissue slices using the genetic GCaMPS reporter line developed by Dr. Bergles. 2-photon microscopy will reveal how the absence of Stimi or Orai results in changes of Ca2+ in cell compartments including spines, dendrites and nuclear envelope. Mechanistic studies will link Ca2+ dynamics with changes in synaptic function and plasticity. These studies are directly relevant to the goals of NIMH since NMDA and calcineurin pathways are important targets for understanding schizophrenia. Aim 2 will explore the role of STIMI-Orai in diseases linked to Ali amyloid. Preliminary studies indicate that deletion of Stimi or Orai results in increased AU-amyloid generation in vivo. This finding, together with previous reports that mutations of presenilini or presenilin2 linked to familial Alzheimer's disease inhibit CCE. suggest that inhibition of CCE may be central to mechanisms of increased AB generation. Studies will define the role of STIMI. Orail and Orai2 in Ali generation, and determine how presenilin inhibits STIMI-Orai coupling to mediate CCE. Since Ali acts to influence synaptic plasticity, an integrated understanding ofthe role of STIMI and Orai in Ali generation and NMDA plasticity will define novel pathways important for synaptic function and diseases of cognition.